The fuel cell stack assembly(ies) of most fuel cell power plants is fueled by hydrogen, or a hydrogen-rich fuel. The sources of such hydrogen are numerous and may include derivation from water, as by hydrolysis, or from solid storage, as from hydrides, but the most common forms are via hydrogen-rich gasses. Perhaps the most common source is from the hydrogen contained in the hydrogen-rich methane (CH4) of natural gas. Other hydrocarbons, such as propane, butane, methanol, ethanol, and various petroleum derivatives, including reformulated gasoline, are also good sources of hydrogen. Still further, other types of organic materials may be processed, as by active digestion and/or natural decomposition, to yield hydrogen-rich fuels, typically grouped as bio-gasses. An example of this latter fuel source is the active digestion of various types of plant and/or animal waste, such as bagasse and the like, via an anaerobic digester to provide “anaerobic digester gas” (ADG), which has a useful content of hydrogen (H2) in the form of methane. Land-fill gas is an example of a bio-gas resulting from natural decomposition, and having a usable H2 (methane) content.
Typically, most or all of the aforementioned sources of hydrogen require some degree of fuel processing at the fuel cell power plant to provide the desired hydrogen-rich fuel stream for the fuel cell stack assembly, and to remove or convert undesirable carbon compounds, such as carbon monoxide. Moreover, while each of the aforementioned fuel sources has a useful hydrogen content, it may vary considerably from one fuel type to another, and in some instances, even within the same fuel type. For example, the hydrogen content of natural gas, as methane, may be relatively high and constant, compared to that of, for instance, bio-gasses, such as ADG and the like, with the latter being even more variable as the result of changes in the feedstock being digested, the temperature of the digestor(s), solids content in the water, etc. Other bio-gasses such as land-fill gas may have an even lower hydrogen content.
Historically, most fuel cell power plants have been designed to operate with a single, particular one of the aforementioned sources of hydrogen-rich fuel, as determined by various factors including performance, availability, and cost of fuel. More recently, some fuel cell power plants have been designed to operate alternately from one or another of multiple fuel sources. Indeed, the 200 kw UTC Fuel Cells PC25™ fuel cell power plant offers the ability for its included fuel processing system (FPS) to accept and process multiple fuel sources, either exclusively or as a blend or mix, as depicted generally in the functional system schematic of FIG. 1.
Referring to FIG. 1, a fuel cell power plant 10 includes a fuel cell stack assembly (CSA) 12, a fuel processing system (FPS) 16, and a controller 18, as well as other components (not shown). The FPS 16 may typically include components of known design for reforming the raw fuel to separate the hydrogen, as by reforming, and for further reacting the reformate, as by shift conversion and/or selective oxidation, to reduce the CO level and relatively increase the H2 level in the fuel stream. The CSA 12 includes an anode 14 that receives a hydrogen-rich fuel stream, via conduit 20 from the FPS 16, for use in the well-known electrochemical reaction(s) that convert the fuel reactant and an oxidant reactant to electrical energy. Multiple fuel sources of different types are represented by at least Primary fuel supply 22 and Alternate #1 fuel supply 24, and may further include Alternate #n fuel supply 26 to represent one or more further fuel sources. Each of the fuel sources 22, 24, and 26 is connected to the FPS 16 via a respective conduit 28, 30 and 32 that contains a respective control valve 34, 36 and 38. The conduits 28, 30, and 32 typically merge downstream of the control valves 34, 36, and 38 at a mixing junction 37 to mix or blend the fuel, and the mixed fuel is then conducted to the FPS 16 via conduit 39. Independent control of each of the control valves 34, 36 and 38 by controller 18 is represented, for simplicity in this Figure, by a single line 40, which may in fact be separate, independent control leads to each valve or may be a single lead for conveying multiplexed signals to the appropriate valves. The controller 18 may be discrete circuitry, a programmable computer, or a combination thereof, and provides control for at least the portions of the power plant 10 associated with fuel delivery and fuel processing. An operator-pre-selected fuel mix value for controller 18, expressed as a percentage of the alternate fuel(s), is represented by control input line 41 to the controller 18, which in turn is operative to control the valves 34, 36, 38 to provide the pre-selected relative percentages of the fuels to be mixed. Typically, the standard (i. e. primary) fuel has been natural gas, with the alternate fuel being another fuel as mentioned above. The operator either selected 0% of the alternate fuel, 100% of the alternate fuel, or a mix comprising some specified percentage of the alternate fuel and the remainder being natural gas. Further, lines 42, 44, and 46, extending via lead 48 (shown in broken line) to controller 18, indicate the status of the respective fuel supplies 22, 24, 26, but typically only in a 2-state fashion, such as on or off, or available or unavailable. The broken-line function block 50 represents signals to and from other controls (not shown), and conducted via the lines 51 and 52 shown in broken-line form.
Although the system described above with reference to FIG. 1 has the capability to use alternate fuels, either separately or in some pre-selected mixed combination, the particular selection of fuels and the scheme for determining their relative amounts has not been addressed. Moreover, there is no suggestion of accommodating variations in the hydrogen content of the fuels during operation. In this latter regard, if the hydrogen content of one or more of the fuels in a mix of multiple fuel sources is sufficiently variable during operation, failure to accommodate or compensate for such variation may cause the CSA 12 to shut down or reduce power in the event the hydrogen content is too low during a “variation”.
At this point it will be noted that while it is the hydrogen content of the fuel stream that is of ultimate concern, it will be convenient to refer to fuels, or fuel components, that have an “equivalent” hydrogen content. For instance, the methane in natural gas is a good source of hydrogen and has a relatively constant equivalent hydrogen content. Similarly, other hydrogen-source fuels have respective equivalent hydrogen contents, though as noted above they may be variable. Indeed, that equivalent hydrogen content may even be expressed as “energy content”, particularly when referring to the heating value of a fuel. For the purposes of the description I the present application, while it is ultimately the hydrogen content of the fuel that is of importance, reference may be made herein to a specific fuel, such as natural gas, ADG, methane or the like, for which there is an “equivalent” hydrogen content, either known or unknown, constant or variable. In such instance, reference to a particular fuel is intended to be a reference to the equivalent hydrogen (equivalent H2) content of that fuel.
For example, the fuel cell power plant 10 described above typically requires a hydrogen content or level that corresponds with at least a 60% methane level in the fuel source(s) in order to maintain full power. If the hydrogen content in the raw fuel is in a form other than methane, such as propane, butane, methanol, etc., then the required level may be other than the 60% methane equivalent and may or may not be expressed as a methane equivalent, but will nevertheless ultimately be characterized and quantified by its equivalent hydrogen content. In any event, this may be of particular concern for ADG and other bio-gasses, which have a relatively low and variable methane, and thus H2, content. While there may be significant advantages, such as cost, etc., to the use of ADG as the sole fuel source, the aforementioned limitations may preclude that. Alternatively, the use of another fuel source in combination with the ADG continues to present some problems because of the uncertainty of the actual level of equivalent H2 content of the ADG at any particular time. This latter problem exists with respect to any combination of fuel sources where the H2 equivalent content of at least one of the fuels is not known and/or is variable. In that same regard, it may be desirable to mix two or more fuels so as to minimize usage of one of the fuels for reasons of cost, carbon minimization, etc., and the variability of H2 content of at least one of the mixed fuels impedes an effective utilization.
Accordingly, it is a principal object of the invention to provide an arrangement in a fuel cell power plant for mixing fuels in a manner that provides normal reliable operation of the fuel cell assembly, such as cost effectiveness, minimization of carbon formation, and/or others.
It is a further object of the invention to provide a control arrangement for mixing fuels in a variable manner to accommodate and/or automatically adjust for, hydrogen content of the fuel.